Currently, commercial services utilizing satellite navigation systems are expanding considerably. Products operating on the basis of radiolocation signals have become widely accessible in everyday home routine within motor vehicles for aiding road navigation initially and more recently within mobile telephone devices for a multitude of personal services. Upgrades to future satellite positioning systems, for example the GALILEO European system, promise much higher performance than current systems. Thus new services which were not able to be envisaged for lack of sufficient reliability and positioning precision are today conceivable for companies, notably road transport and air transport companies. For example, for road transport, efforts are under way to transform the economic models of the services for operating toll road sections by offering the customer personalized offers. For air transport companies, increased performance in terms of reliability and positioning precision allows the integration within aircraft of navigation devices on which pilots will be able to rely entirely. These devices will make it possible to considerably improve air transport safety. However, for services on which people's safety depends, it is mandatory to prove the reliability of the data sent by the navigation system. This is why operators of satellite navigation systems are made subject by the authorities to requirements regarding guaranteed service to the end customer.
Satellite navigation systems are characterized by performance data relating to integrity, precision and coverage. Integrity is a measure of confidence in the information provided by the satellite navigation system. A well known tool for determining the integrity of a point provided is the Stanford chart. The Stanford chart is a two-dimensional matrix whose input parameter on the horizontal axis is the observed position error vertically or horizontally and whose input parameter on the vertical axis is the protection level vertically or horizontally calculated on the basis of statistical models. This chart makes it possible to verify the proportion of measured samples whose observed position error is lower than the protection level.
The precision of a position is defined by the position error estimated with respect to the actual position. The precision of the location depends notably on the error in the estimated distance between the user and the satellites received as well as on the configuration of the geometry of the measurements. There exists a value, commonly called the DOP for “Dilution of Precision”, which is indicative of the conditions of geometry of the measurements. When the value of the DOP is high, this indicates that the satellites used to obtain the position are close and therefore that the geometry is bad and when the value of the DOP is low this indicates that the satellites used to obtain the position are distant and therefore that the geometry is good.
The bodies responsible for regulations and checks relating to civil aviation require rigorous levels of performance notably as regards precision performance for critical services. Among these critical services utilizing the geo-location data of satellite navigation systems, the LPV200 service (“Localizer Performance with Vertical Guidance”) required in the past that the satellite navigation system show for at least 95% of the time a location error vertically of less than 4 metres and horizontally of less than 16 metres. In the future, operators will be required to prove that the satellite navigation system proves to the user a probability of occurrence of a location error vertically of greater than 10 metres of less than 10−7 under normal conditions and a probability of occurrence of a location error of greater than 15 metres of less than 10−5 under deteriorated conditions. This service defines the alert level vertically at 35 metres and horizontally at 40 metres.
It is known that augmented satellite systems are capable of complying with the specifications demanded for events of very low probability. These verificatory checks are being performed through unwieldy and irksome methods during the development phases. According to current techniques, they would require the carrying out of measurements for which the time taken for the test would be of excessively long duration (i.e. several tens of years). Indeed, to carry out measurements of integrity margins, classical inferential statistics seeks to model the behaviour of a random variable over the observable domain of realizations. To obtain relevant statistics, it is necessary to recover data which are sufficiently uncorrelated so as not to measure redundant information. It is estimated that it is necessary to carry out samplings with a period of around 5 minutes between each measurement. However, given the low probability of the events that one seeks to detect, this would involve gathering thousands of millions of samples over thousands of years of measurements.
In the earlier patent application WO/2009/112483, the Applicant has disclosed a device providing the means for estimating an indication of integrity of a satellite navigation system making it possible to model the distribution of location errors of very low probability on the basis of extreme value theory. However, no tool for measuring precision currently exists which allows levels of requirement at low probabilities of occurrence to be certified to the user since the precision performance also depends on the satellite geometry, and the data collected do not take into account all cases of satellite geometry for each user.